1. Field of the Invention
This invention relates to an image reading apparatus, and, for example, to an image reading apparatus which can read a projection image magnified from a microfilm, perform image processing, such as contrast correction and the like, and output the resultant image signals to an image forming apparatus, such as an LBP (laser-beam printer) or the like.
2. Description of the Related Art
A configuration has been known wherein an image recorded on a recording material, such as paper or the like, or on a film, such as a microfilm or the like, is photoelectrically read, image recording is performed according to image signals obtained by the reading, and the image signals are, for example, stored.
In such a configuration, in order to always obtain excellent image signals irrespective of the density and contrast of an image to be read, the density and contrast of the image are measured in advance, and the threshold value used for binary-coding the image signals, a parameter for density correction, the amount of exposure of the image, and the like are controlled according to the result of measurement.
The assignee of the present application has proposed image reading apparatuses which have a function to determine the threshold value for a binary-coding operation according to the result of measurement of the density and contrast of an image, in, for example, U.S. Pat. Nos. 4,907,286 and 4,837,450.
For example, in U.S. Pat. No. 4,837,450, a magnified projection image of a microfilm is read by a line sensor, such as a CCD (charge-coupled device) or the like, and a density histogram is formed from image signals obtained by the reading. A threshold value for a proper binary-coding operation is determined from the density distribution of the histogram, and binary-coded image signals are output to an image forming apparatus.
FIG. 7 illustrates a density histogram thus obtained. In general, a negative film is mostly used as a microfilm. Hence, density distribution portion 101 corresponds to a base (background) portion, and density distribution portion 102 corresponds, for example, to character portions. If the density data having a peak in frequency in the distribution portion 101 is represented by P.sub.b1, and the density data having a peak in frequency in the distribution portion 102 is represented by P.sub.w1, the threshold value T.sub.1 is expressed by the following expression: EQU T.sub.1 =(P.sub.b1 +P.sub.w1)/2 (1).
When an original is photographed on a microfilm, an image fogged over the entire region of the film is obtained in some cases due to improper exposure time or developing process. In such a case, the density histogram shifts to the black side as a whole, as shown in FIG. 8A. When such image data are subjected to density adjustment processing with 256 gradations, the maximum value D.sub.max1 in the data is obtained, and respective data are multiplied by (256/D.sub.max1). Thus, a density histogram as shown in FIG. 8B is obtained. By performing such contrast correction, the threshold value T.sub.2 can be obtained in the same manner as described above: EQU T.sub.2 =(P.sub.b2 +P.sub.w2)/2 (2).
When a diazo film is used as a microfilm, remarkable fading occurs due to natural light, transmitted light in a reader printer or the like. Hence, the density histogram shifts to the white side as a whole, as shown in FIG. 9. When processing such image data with 256 gradations, the minimum value D.sub.min2 and the maximum value D.sub.max2 of the data are obtained, the value D.sub.min2 is subtracted from respective data D.sub.n2 between the above-described two values D.sub.min2 and D.sub.max2, and the resultant values are multiplied by {256/(D.sub.max2 -D.sub.min2)}. Thus, a density histogram as shown in FIG. 8B is obtained. By performing such contrast correction, the threshold value can be obtained in the same manner as in the above-described two examples.
In the above-described examples, however, in order to determine the threshold value, the peak value Pb of the distribution portion corresponding to base (background) portions and the peak value Pw of the distribution portion corresponding to character portions must be present. In some cases, however, only one peak value is present, as shown in FIG. 10, or three or more peaks are present, as shown in FIG. 11.
The above-described cases may occur if many halftone portions are present as in a photograph or the like, or if characters are inserted in part of a photograph, or a frame is present around an image. The above-described approach has the following disadvantages: When constrast is low as in the above-described cases, the threshold value cannot be determined by the above-described method. Even if peak values are present, it is very difficult to determine a proper threshold value. Binary-coded image signals obtained from temporarily-determined threshold value are hard to be read, or in some cases impossible to read.